1. Field of the Invention
This invention relates to a method of detecting an overcurrent flowing in a photomultiplier and protecting the photomultiplier therefrom, and more particularly to an overcurrent detection and protection method for a photomultiplier used in an image scanning and read-out method in which an image recorded on a sheet is read out by two-dimensionally scanning the sheet with a light beam such as laser beam, and detecting the light obtained from the sheet in the pattern of the recorded image by use of the photomultiplier.
2. Description of the Prior Art
Heretofore, there has been widely used a method of scanning and reading out an image in which an image recorded on a sheet is read out by two-dimensionally scanning the sheet with a light beam such as laser beam, and detecting the light (for example, reflected light or transmitted light) obtained in the pattern of the recorded image by the exposure of the sheet to the light beam by use of a photomultiplier.
Such an image scanning and read-out method using a photomultiplier is employed, for example, in an input apparatus for a computer, an image read-out apparatus for facsimile telegraphy, and the like. In the apparatus of this type, image read-out is specifically conducted as described below.
Namely, a sheet carrying an image composed of a density pattern, i.e. an original, is exposed to and two-dimensionally scanned with a light beam, and the reflected light (in the case of a paper original) or transmitted light (in the case of a film original) thereby obtained is detected and converted into a serial electric image, thereby to read out the image recorded on the original. The electric signal obtained can be processed in various ways, for example, transfer to a remote place by a transfer system, image processing, storage in a magnetic recording medium, and the like.
The aforesaid image scanning and read-out method using a photomultiplier is used also in a radiation image system using a stimulable phosphor sheet, which is disclosed, for example, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395. In this system, a stimulable phosphor sheet carrying a radiation image stored therein is exposed to stimulating rays to sequentially release the radiation energy as light emission, and the light emitted upon stimulation of the sheet is photoelectrically read out.
The stimulable phosphor referred to in this invention means a phosphor which is able to store a part of radiation energy when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, and then emits light in proportion to the stored energy of the radiation when exposed to stimulating rays such as visible light.
In the aforesaid radiation image system using a stimulable phosphor sheet, image read-out is specifically conducted as described below.
Namely, a stimulable phosphor sheet carrying a radiation transmission image stored therein by exposing the stimulable phosphor sheet to a radiation such as X-rays passing through an object such as human body is two-dimensionally scanned with stimulating rays such as laser beam to sequentially release the radiation energy stored in the stimulable phosphor as light emission. Then, the emitted light is photoelectrically read out by a photomultiplier to obtain an image signal, thereby to read out the image stored in the stimulable phosphor sheet. Based on the obtained image signal, a visible image may be reproduced on a recording material such as photographic light-sensitive material, a cathode ray tube (CRT), or the like.
This radiation image system using the stimulable phosphor sheet is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed in various ways to improve the image quality for viewing, particularly diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the stimulable phosphor varies over a wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording material such as photographic light-sensitive material or on a display unit such as CRT. The electric signal may further be processed in various ways as desired to obtain a radiation image suitable for viewing, particularly diagnostic purposes. This is very advantageous in practical use.
However, the image scanning and read-out apparatus based on the aforesaid image scanning and read-out method using a photomultiplier as a photodetector presents a very real problem as described below.
The photomultiplier is suitable for detecting very weak light. However, the photomultiplier has a drawback that, when the photomultiplier is exposed to strong light (i.e. light so strong as to be visually seen bright) with a voltage applied thereto, an excessive photoelectric current flows therethrough, and the photomultiplier breaks if this condition continues. Therefore, the image scanning and read-out apparatus based on the aforesaid image scanning and read-out method using the photomultiplier is so constructed that the apparatus is completely shielded from external light in order to prevent the photomultiplier from being broken due to light entering thereto from the outside of the apparatus.
However, it sometimes happens that the shielding of the apparatus from external light is incomplete due to a defective assembly process for the apparatus or due to a failure to close a portion of the apparatus opened for adjustment, maintenance or the like. If a voltage is applied to the photomultiplier in order to start the apparatus in this condition, the photomultiplier readily breaks due to external light entering the apparatus. Actually, this problem occurs very frequently, and makes the aforesaid image scanning and read-out method very uneconomic since the photomultiplier is very expensive.
One solution to the aforesaid problem is to protect the photomultiplier by stopping the voltage supply to the photomultiplier when the output current obtained from the photomultiplier exceeds a predetermined value. However, this solution is practically not preferable since the voltage supply to the photomultiplier is stopped even when there is no external light entering the apparatus, i.e. when the intensity of the reflected light, transmitted light or the light emitted from the stimulable phosphor upon stimulation thereof, which is obtained by exposing the sheet carrying an image recorded thereon to a light beam in order to read out the image, becomes larger than the aforesaid predetermined value according to the actual level of image density.